1. Field of the Invention
The present invention relates to a method of detecting the origin of a shear and measuring the amount of a shear in a shearing interferometer system.
2. Discussion of Background
The shearing interferometric system is known as a system for measuring with high accuracy the surface configuration of an object and the performance of a lens. In this system, light bearing information about an object being measured, such as light reflected from a surface of an object being measured or light that has passed through a lens being checked for its performance, is referred to as measuring light.
The measuring light is divided into a basic light beam and a reference light beam which is laterally shifted with respect to the basic light beam. The interval by which these light beams are laterally shifted with respect to each other is referred to a shear.
The two light beams are equivalent to each other and have the same wavefront. For the sake of simplicity, one-dimensional measuring principles will be described below. Assuming that the wavefront shape of the basic light beam on an area sensor is expressed by W(x), the wavefront shape of the reference light beam on the area sensor is expressed by W(x+S) due to the shear S. Since the shear S is generally quite small, the difference between the wavefronts W(x+S)-W(x) becomes: ##EQU1## and can be expressed as .DELTA.W(x).S if ##EQU2## The value W(x) can be known with high accuracy by measuring and analyzing the interference fringe pattern of two light beams in a known fringe scanning interferometric system. By effecting an arithmetic operation: ##EQU3## the wavefront shape W(x) can be determined. Then, the wavefront shape is corrected on the basis of a correlationship between the wavefront shape and the configuration or the like of the surface being measured, so that the surface configuration can be identified. Information on the performance of a lens can be obtained from the wavefront shape thus determined.
In order to effect accurate measurements in the shearing interferometric system, therefore, the righthand side of the above equation (1) must be accurately computed. The accuracy of the righthand side of the equation (1) is directly affected by the shear S, and hence the accuracy of the shearing interferometric system itself is largely dependent on the accuracy of the shear S. For sufficient measuring accuracy, the accuracy of the shear S should preferably be kept to 0.1 micrometer. Since the shear S is a shift between the reference light beam and the light beam shifted therefrom, it is necessary that the condition in which no shear is present, i.e., the origin of the shear be accurately be measured for exact determination of the shear.
Heretofore, the origin of a shear has been detected in the following manner: In the origin of a shear, the basic and reference light beams are completely overlapped with no phase difference, and hence there is no interference fringe formed on an area sensor. Therefore, it has been customary to display the interference fringe on a CRT based on the output signal from the area sensor and adjust the shear to zero while monitoring the displayed interference fringe for detecting the shear origin.
However, the conventional practice is not necessarily easy to perform, and the detection accuracy is largely governed by the experience and skill of the operator.
It would be convenient if the shear S could be measured directly with high accuracy.